A glass substrate for displays such as liquid crystal display devices is required not to substantially contain alkali metals, and therefore, alkali-free glass is used for such a glass substrate. Further, such a glass substrate is required to have high chemical resistance and durability, to contain little bubbles in the glass and to have high homogeneity and high flatness.
A glass raw material for alkali-free glass does not substantially contain alkali metal compounds, and such a glass raw material is hardly meltable. Therefore, it has heretofore been said that as silica sand being the main component of the glass raw material, it is necessary to employ one having a small particle diameter. However, if a glass raw material containing silica sand having a small particle diameter was melted, there was a case where fine particles of silica sand tended to agglomerate one another to form coarse secondary particles, whereby the glass raw material was not completely melted.
Further, in order to impart the melting properties, chemical resistance and durability to alkali-free glass, B2O3 may be incorporated to a glass composition. As a raw material for such B2O3, orthoboric acid (which may also be simply called boric acid) is used, since it is inexpensive and readily available. However, if a glass raw material containing orthoboric acid is employed, fine particles of silica sand tend to be more readily agglomerated. Thus, there was a case where the temperature of molten glass in a melting furnace became unstable, or circulation/residence time of molten glass became unstable.
Once agglomeration of fine particles of silica sand occurs, the homogeneity of molten glass deteriorates, and consequently, the homogeneity and flatness of formed alkali-free glass tend to be low. Further, if the circulation/residence time of molten glass in a melting furnace becomes unstable, there may be a case where a part of molten glass flows out of the melting furnace before bubbles are withdrawn by a clarifier from the molten glass in the melting furnace. Further, melting of the glass raw material is non-uniform, and the effects of the clarifier to silica sand melted belatedly, tend to be inadequate, whereby there may be a case where bubbles are not sufficiently withdrawn from the molten glass.
For the purpose of improving the homogeneity of alkali-free glass, glass raw materials have been proposed wherein the particle diameters of alkaline earth metal compounds (strontium carbonate and dolomite) are controlled (Patent Document 1). However, in the glass raw materials disclosed in Patent Document 1, no consideration is given with respect to silica sand which is melted belatedly. There has been a case where if melting of silica sand delays, silica sand in a non-melted state is captured by bubbles formed in molten glass and gathers in the vicinity of the surface of the molten glass, whereby there will be a difference in the composition of the SiO2 component between the surface layer and other portions of the molten glass, thus leading to a deterioration of the homogeneity of glass.
Patent Document 1: JP-A-2003-40641